Heater bar system



HEATER BAR SYSTEM Filed Sept. 23, 1965 l N VliN 'I 0R5 W\LLIAM I. DIETERS W BERNAR H.PADDOCK 4'5- 27 -1 ATTORNEY United States Patent 3,465,812 HEATER BAR SYSTEM William I. Dieters, Arlington Heights, and Bernard H. Paddock, Wheeling, Ill., assignors to Martin-Marietta Corporation, New York, N.Y., a corporation of Maryland Filed Sept. 23, 1965, Ser. No. 489,474 Int. Cl. B22d /00; B22c 9/22, 9/08 US. Cl. 164-361 3 Claims ABSTRACT OF THE DISCLOSURE A mold and process of casting wherein metal cast into a mold is frozen simultaneously with additional portions of the metal in walled passages positioned external to the mold and in heat transfer relationship to portions of the mold bounding volumes of small effective section size. By this means uniform grain sizes can be provided across varying sized sections of the cast object.

The present invention is concerned with casting of metal and, more particularly with methods and apparatus adapted to be employed in precision casting of complex shapes having non-uniform section size.

It is well known that the internal structure of a cast metal is dependent upon many factors including, but not necessarily limited to, metal composition, temperature at pouring, rate of solidification, direction of solidification, type of inoculation (if any), section size, feeding, risering and the like. The foundry industry in general has developed techniques for each different type of casting process which techniques have been fairly successful overall. It has been found, however, that the techniques usually applied in the precision casting of structures having a diversity of section sizes are not totally satisfactory for producing articles such as gas-turbine blades to the tolerances relating to grain size and grain configuration which are demanded by the gas-turbine industry.

More specifically, it has been found that the required control of the grain size and grain configuration is very difficult to achieve when section sizes in a single object differ by more than a ratio of about 3 to 1. For example, when a turbine blade is made by precision casting, using the best techniques known to the prior art, it is often found that lack of grain size control exists with respect to the internal and/or external structure of the turbine blade in the area adjacent the thin trailing edge of the blade. It is a general requirement of the gas turbine manufacturers that there be a uniform grain size on airfoil surfaces of a cast turbine blade. Such is not ordinarily obtained when a turbine blade is cast in the usual manner using a fugitive model which is a duplicate in size and configuration (allowing for casting shrinkage) of the desired turbine blade. Attempts to beef up the thin trailing section of the as-cast blade and then grind or machine to final shape have been unsuccessful not only because of the high costs and high rejection rate involved in the grinding and/or machining steps, but also because such operations inherently can cause deleterious incipient micro cracks in the alloys normally employed in the manufacture of cast turbine blades. As far as We are aware, this problem has not been solved heretofore by any precision casting techniques carried out successfully on a commercial scale.

It has now been discovered that by means of special mold configurations, or, as viewed in a different fashion, that by means of a special process the aforedescribed problem can be successfully overcome as a practical matter.

3,465,812 Patented Sept. 9, 1969 It is an object of the present invention to provide a novel casting process.

Another object of the present invention is to provide a novel casting means.

An additional object of the present invention is to provide a novel process of precision casting.

A further object of the invention is to provide improved means for precision casting.

A still further object of the present invention is to provide a novel shell mold for precision casting.

Other objects and advantages will become apparent from the following specification taken in conjunction with the drawing in which:

FIGURE 1 is a view in cross section along plane II of a ceramic shell mold for precision casting a turbine blade in accordance with the present invention;

FIGURE 2 is a view in cross section along plane II-II of the ceramic shell mold depicted in FIGURE 1;

FIGURE 3a is a representation of the external and/or internal grain structure of the trailing portion of a turbine blade made in accordance with the present invention;

FIGURE 3b is a representation of the external and/ or internal grain structure of the trailing portion of a turbine blade made without the benefit of heater bars as employed in the practice of the present invention; and

FIGURE 4 is a cross-sectional view of a compacted mold adapted to produce a triangular shaped bar in accordance with the present invention.

Generally speaking, the present invention contemplates casting metal in a mold having at least one section thinner than the remaining sections and freezing the metal therein under conditions such that an additional portion of the metal being cast is freezing in a position exterior to' and adjacent a wall of the thinner section and in heat transfer relationship therewith. The present invention is particularly applicable to the production of cast objects by the precision casting process wherein a shell-type mold is employed either per se or invested in a ceramic investment. Generally speaking, the apparatus employed in accordance with the present invention comprises a hollow mold with differing effect section sizes. At least one, and, advantageously, a plurality of walled passages are in communication with the interior of the mold and lead oflf from non-critical portions of the mold. These passages which are exterior to the mold proper follow a path such that at least a part of the exterior surfaces of the passages are in heat transfer relationship to a surface of the mold proper which bounds a volume of small effective section size. When metal is poured into the mold it fills both the mold proper and the exterior passages. As the metal freezes, heat is emitted both from the mold proper and from the filled passages. The close positioning of the walls of the passages and selected portions of the walls of the mold proper causes a reduction in the rate of freezing of thin section metal and a consequent uniformity of grain structure in the cast object.

For purposes of this specification and the appended claims, it appears advisable to define or clarify certain terms employed herein. Thus, it is to be observed that hollow mold is intended to include molds which contain inserts of various types but which are truly hollow in that they provide a volume into which molten metal may be cast. The term mold as employed herein includes any type of mold which is built up upon a form to a self-sustaining thickness, subsequently removed from the form (or vice versa) and treated, if necessary, to achieve suflicient strength to contain molten and/or frozen metal. Heat transmission includes for purposes of this specification all forms thereof, to wit: conduction, convection and radiation. In vacuum casting into a mold which is an area of high utility for the present invention,

cooling of the freezing temperatures of cobalt-base, nickel-base and iron-base alloys is primarily by radiaation from mold wall surfaces in the absence of chill devices. When casting into a massive mold (such as a sand mold or an invested mold) conduction becomes an important means of heat transmission. Regardless of the mechanism of heat transmission, for purposes of this specification and the appended claims, two surfaces shall be considered to be within heat transfer relationship to each other so long as they are spaced no further apart than twice the minimum thickness of the larger section enclosed thereby. In the usual case, the external passages (referred to hereinafter as heater bar passages) will be circular or quasi-circular in cross section and will have a larger section thickness (i.e., diameter) than the surfaces to which they are adjacent. Hence the heater bar passage surfaces and the adjacent surfaces shall be considered to be in heat transfer relationship if the surfaces in question are spaced apart by a distance no greater than twice the minimum outside diameter of the heater bar passages.

The present invention is particularly applicable to casting of heat-resistant alloys and refractory alloys. Heatresisting alloys include nickel-base alloys, cobalt-base alloys and iron-base alloys which contain correlated and interrelated amounts of elements such as chromium, tungsten, molybdenum, titanium, aluminum, niobium, tantalum, zirconium, boron and may contain incidental and/ or effective amounts of elements such as manganese, silicon, rare earth metals, calcium and the like. As a general rule, heat resistant alloys are melted and cast at temperatures in excess of about 2,500 F. under some protective means, advantageously under high vacuum.

In order to describe the present invention with greater particularity, attention is directed to the drawing. Referring now thereto and more specifically to FIGURES 1 and 2, it is to be observed that FIGURE 1 is a view in elevation and FIGURE 2 is a view in plan of ceramic shell mold 11. Ceramic shell mold 11 is made usually by dipping a wax or other fugitive model of the object to be cast in successive layers of hardenable ceramic slip casting material. Suitable slip casting materials include zirconium oxide, aluminum oxide, thorium oxide and other oxidic materials stable at a high temperature. The initial green strength and final fired strength of ceramic shell mold 11 is obtained by the incorporation of small amounts of collodial silica, sodium silicate or ethyl silicate as a binder in the slip cast composition. In the specific instance depicted in FIGURES 1 and 2, ceramic shell mold 11 is made by slip casting successive layers of ceramic material on a wax model of a turbine blade. After the ceramic material has set and dried, the wax model is removed to provide root cavity 12, shroud cavity 13 and airfoil cavity 14 together with runner 15, riser 16, heater bar cavity 17. With respect to requirements as to uniformity of cast metal grain size, airfoil cavity 14 is the critical portion of shell mold 11. It is to be observed that for clarity in illustrating the present invention, other conventional portions of a refractory shell mold have been omitted. Thus, one skilled in the art will recognize that runner 15 will open upon a gate and a sprue through which molten metal can be poured. Likewise, riser 16 will take on various forms which are well known to those skilled in the art. Also, as is well known to those skilled in the art, prior to metal being cast into shell mold 11, shell mold 11 must be fired to achieve its ultimate strength and, advantageously, as set forth and claimed in US. Patent No. 3,153,824, may be vacuum treated at temperatures in excess of the pouring temperature of the metal to be cast while being held under high vacuum. Critical to the operability of the present invention is the positioning of heater bar cavity 17 and heater bar cavity wall 18 defining cavity 17. It is to be observed that heater bar cavity 17 communicates with the main portion of the interior of ceramic shell mold 11 through walls 19 which defines a portion of the root cavity 12 and through wall 20 which defines a portion of the shroud cavity 13. Over a major portion of the length of heater bar cavity 17 a portion of wall 18 is positioned adjacent to and in heat transfer relationship with that portion of shell mold 11 which encloses the thin trailing portion of airfoil cavity 14. In FIGURE 2 the sections of the wall 18 and shell mold 11, which are in heat transfer relationship, are particularly pointed out by reference numeral 21. After preliminary treatments, as described hereinbefore, are completed, metal is poured into shell mold 11 through runner 15 into root cavity 12, airfoil cavity 14, shroud cavity 13 and riser cavity 16. Simultaneously, molten metal enters heater bar cavity 17. Premature chilling and freezing of the metal in shell mold 11 in the areas adjacent filled heater bar cavity 17 is inhibited by heat transmitted from the metal within heater bar cavity walls 18. When, as is advantageous, the casting is conducted in vacuum without any massive investment surrounding shell mold 11, the hot walls 18 of the heater bar cavity provide a partial high temperature environment for the sections of shell mold 11 to which it is adjacent. This inhibits the local transfer of heat by radiation from selected portions of shell mold 11 in accordance with the well-known principles of the Stefan-Boltzman law. The net effect of this is to even out variation in cooling rate across the varying section of shell mold 11 and thus provide uniformity of crystal structure across the metal filling airfoil cavity 14. The advantageous results of the present invention are depicted at 22 in FIGURE 3a in direct comparison to the results heretofore obtained by the prior art as depicted at 23 in FIGURE 3b.

FIGURE 4 illustrates that the present invention is not limited to that instance where metal is cast into an unsupported shell mold. Referring now to FIGURE 4, it is to be observed that packed mold material 24, e.g., sand or solidified investment, surrounds cavity surface 25 defining a triangular shaped cavity and heater bar cavities 26. The cavities defined by walls 25 are fed from sprue 27 through gates and runners not shown. In this some what academic illustration, it is assumed that the apices of the triangular cavity adjacent heater bar cavities 26 will ultimately contain metal, the crystal structure of which is critical to the operability of the cast structure. The triangular shape of the major cavity defined by walls 25 was selected to illustrate the fact that at times it is difficult to defined sections size in precise terms. From inspection, however, it is clear that at the apex of the triangular shape cavity there is more effective cavity wall area per unit of immediately enclosed volume than there is at the mid-point of any cavity wall defining a side of said triangle. Therefore, one can say that the effective section size of the triangular cavity is small near the apices and large near the mid-points of the Wall defining the triangular shape. Accordingly, cavity surfaces 25 adjacent two apices and the opposing surfaces 25 of the adjacent heater bar cavity walls are placed in heat transfer relationship 21 in order to achieve the purposes and advantages of the present invention. Thus the present invention is applicable to any casting which, by virtue of either the dimensions or the geometry thereof, exhibits sections which effectively differ in size and which exhibit a deleterious differential in cooling rate.

In carrying out the present invention it is advantageous to interrelate the sectional size of the heater bars and the sectional sizes of the object being cast. In general, the total section thickness of heater bars surrounding or adjacent a mold surface should be in the range of about 1 to about 3 times the difference in section thickness between the average section thickness of object being cast (omitting from said averaging the thin section in question) and the average thickness of the thin section. Mathematically this relationship can be expressed as:

T HB=K(TA' T T) where T =total thickness of heater bars K=a factor within the range of 1 to 3 T =average thickness of thicker sections of object to be cast and T =average thickness of thinner section of object to be cast.

For exemplification this computation is applied to the production of the airfoil section depicted in FIGURE 2 of the drawing:

T =K(3.75 mm.)=about 3.75 mm. to about 11.25 mm.

In instances where a product of substantially uniform cross section is being cast, for example, a metal strip, and it is found that grain refinement is needed at certain locations such as at the edges, total thickness of heater bars is conveniently determined by the formula T =K TP where T P equals the thickness of the section.

The heater bar thickness is measured on the diameter if the cross section is circular and on the minor axis if the cross section is other than circular. In the ordinary case the larger permissible heater bar sections will be employed when the heat transfer distance between the casting proper and the heater bar is greater and vice versa. While the foregoing computation provides a general rule to guide those skilled in the art in the practice of the present invention, it must be recognized that some variations are to be permitted to account for factors such as the particular geometry of a part being cast, heat transfer conditions and the like.

In accordance with the foregoing teachings, a metal having the following nominal composition: 9% chromium, 12% tungsten, 0.15% carbon, 1% tantalum, 5% aluminum, 2% titanium, cobalt, with the balance essentially nickel, was cast under 'vacuum conditions with mold pretreatment as described in US. Patent No. 3,153,824 into:

(A) a mold as depicted in FIGURES 1 and 2 of the drawing and (B) into a similar mold except that the similar mold contained no heater bar cavities.

The metal cast as per (A) above exhibited the satisfactory structure as depicted in FIGURE 3a of the drawing Whereas the metal cast as per (B) above exhibited the unsatisfactory structure as depicted in FIGURE 3b of the drawing. It is to be noted that the average grain size present in a cast section is dependent upon many factors including, but not limited to, pouring temperature, inoculation and the like. By means of the present invention, a casting can be produced having a substantially uniform grain size, as shown in FIGURE 3a, rather than a structure having differing grain sizes which is produced by prior art methods.

The present invention is particularly adapted for the casting of heat resisting metals such as the alloy specifically disclosed in the foregoing example and for the casting of refractory metals such as tungsten, molybdenum, tantalum, columbium and alloys thereof containing major proportions of one or more of said refractory metals. It is contemplated that the present invention can be employed not only to even out the crystalline habit of cast objects but also to provide directional characteristics when employed in association with chills and like means.

While the present invention has been described in conjunction with advantageous embodiments, those skilled in the art will recognize that modifications and variations may be resorted to without departing from the spirit and scope of the invention. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

We claim:

1. A casting means comprising a hollow mold structure defining an elongated cavity, means for feeding molten metal to said cavity comprising at least one runner and riser communicating with said cavity and at least one walled passage exterior to said mold structure, distinct from said feeding means, in communication with said cavity adjacent the ends thereof, said mold structure and said at least one passage being capable of containing molten metal and being capable of transmitting heat from said molten metal to induce freezing thereof, said cavity being of a configuration such that at least a portion thereof has an airfoil-shaped cross section and said at least one walled passage extending substantially parallel to, substantially coextensive with, adjacent to and in heat transfer relationship with that portion of said mold structure which bounds the trailing edge portion of said airfoil-shaped cross section.

2. A casting means as in claim 1 wherein the hollow mold structure is a ceramic shell mold.

3. A casting means as in claim 1 having a plurality of walled passages.

References Cited UNITED STATES PATENTS 3,015,138 1/1962 Watts 249- X 3,204,303 9/1965 Chandley 164-35 X 1,939,963 12/ 1933 Ferlin 249-107 1,732,382 10/ 1929 Schultz 249-107 X 2,266,723 12/ 1941 Fahlman 164-122 2,782,476 2/1957 Brennan 164-338 2,820,266 1/1958 Kohl 164-361 3,091,825 6/1963 Deppeler et a1.

3,103,721 9/1963 Bishop et al 164-53 I. SPENCER OVERHOLSER, Primary Examiner V. RISING, Assistant Examiner 

